1.Field of the Invention
The invention relates to a stylus for tracking a stereophonic and/or quadraphonic sound groove of a record, the stylus consisting of a mounting portion and a taper.
2. Prior Art
Pick up styli with a point shaped like a circular cone with rounded vertex have been known for a considerable time. A drawback in their use has been the fact that the stylus touches the two walls of the V-shaped sound groove of a record at near circular surfaces, resulting in only partial use of the groove wall in the region of the groove wall which lies in a plane perpendicular to the groove of the record. At the same time, with regard to the direction of motion of the record, the stylus touches a relatively large surface of the groove compared with the wave length of the high frequencies (i.e. up to 20 kilohertz). Since the cutter for cutting the sound grooves of the record has sharp edges, (i.e. the smallest possible radii of curvature of about 3 microns) the stylus with its relatively large radii of curvature can not accurately pick up the high frequencies recorded in the record grooves. Additional undesirable effects, such as squeezing of the stylus, binding, emphasis and de-emphasis with double frequency to lateral deflection were very unfavorable for stereo reproduction systems using 45.degree.-45.degree. recording.
In view of the above, a diamond tip stylus was suggested with a taper shaped like a chisel, the two opposing facets forming a chisel-like edge about 35 microns wide. This chisel-like edge is placed at right angles to the longitudinal direction of the sound groove. Considering the direction of motion, the area of contact with the sound groove is very small, making for better playback of high frequencies, but also permanently deforming the record and modifying the original groove-wall-displacements. Bringing the main axis of the rough diamond into line with the longitudinal axis of the stylus increases firmness of the stylus. The pick up stylus is mounted on the cantilever in such a way that the chisel-like edge is perpendicular to the longitudinal direction of the cantilever. Yet, the fact that the two walls of the sound groove of the record were only partially tracked, still resulted in unsatisfactory performance.
To eliminate this drawback, a pickup stylus was developed wherein the cross sections over the length of its taper have the form of ellipses of decreasing size, the curves at the ends of the major axis of the ellipses along at least the active part of the taper having at least approximately equal radii of curvature and the rounding of the free end of the taper, looking in the direction of the minor axis of the ellipses, having a radius of curvature greater than the mentioned approximately equal radi of curvature. Yet, sound reproduction proved to be modestly satisfactory only at low and mid-pickup frequencies.
Consequently, a stylus was developed whose taper has ground sections on two opposing sides so that their edges de-limit narrow facets which partially touch the walls of the sound groove, each pair of such adjoining edges, at least in the area of contact, running approximately parallel to each other, with the result that two tangents drawn along the edges in the area of contact run parallel to each other. The ground sections each consist of two plane facets with an obtuse angle (about 120.degree.) between them, the dihedral angles lying in the plane determined by the transit direction of the stylus axis. These styli result in narrow, relatively short contact areas with the two groove walls of the record.
In general, undistored playback is insured only if the contact areas of the stylus tip edges are very narrow, permitting the stylus to rapidly follow sudden changes in the groove of the record when tracking a recording. In the case of high frequency recordings, however, such a narrow shape of the stylus tip is insufficient for undistorted playback because the movement of the stylus causes variation of the tracking weight on the disc groove with accompanying plastic deformation.
To eliminate this drawback, a pickup stylus was designed with a coniform taper, of which two ground sections with an obtuse angle (120.degree.) between them together form an edge. The vertex on the cone lies in the part to be removed and the intersecting lines of the ground sections cut the axis of the cone at an angle of less than 90.degree.. This angle as well as the dihedral angle of the ground sections are so chosen that those parts of the stylus in contact with the sound groove lie on a plane normal to the groove.
It has now been determined by the applicant herein that the desirable shape for a stylus is one which is capable of tracking a maximum distance along a line which lies on the groove and is in the plane perpendicular to the axis of the groove. It has also been found that the stylus should track over a minimum distance in the direction of groove movement. One prior art stylus which partially solved this problem is the Shibata-type of stylus and this stylus did not solve the problem for other reasons. All of the prior art styli, aside from the shibata-type, do not have a proper radius of curvature, such radius of curvature being either too large or too small, that is, too large in the front-back direction and too small in the vertical direction.
In order to reproduce a maximum of high frequency signals, the prior art styli as described above, utilized an excessive weight to artificially force the styli into the high frequency portions of the grooves, these being the smaller modulation regions. While this increased the high frequency response, this excess weight also tended to damage the records by providing a permanent deformation in the grooves as well as by scraping out portions of the record surface. This was all due to the fact that the prior art styli did not conform optimally to the shape of the groove walls.